booster boost·er (bōō'stər)n. An additional dose of an immunizing agent, such as a vaccine or toxoid, given at a time after the initial dose to sustain the immune response elicited by the previous dose of the same agent. Also called booster dose, booster shot.

(b'stər) An additional dose of an immunizing agent, such as a vaccine or toxoid, given at a time period of weeks to years after the initial dose to sustain the immune response elicited by the first dose. Tetanus, diphtheria, and measles vaccines are commonly given in booster doses.